At the present time, virtually all printed copy is produced through the use of three basic types of printing plates. One type is a relief plate which prints from a raised surface. Another type is an intaglio plate which prints from a depressed surface. The third type is the lithographic plate which prints from a substantially flat surface which is neither appreciably raised above nor appreciably depressed below the adjacent and surrounding non-printing areas. Printing is occasioned by an ink's respective affinity and/or aversion to areas of different chemical properties. Lithographic printing plates are commonly processed to have water-repellent (hydrophobic), oil-receptive (oleophilic) image areas and water-receptive (hydrophilic) non-image areas.
Prior to processing for use, conventional lithographic plates will typically have a hydrophobic, photoreactive polymeric layer (i.e. photoresist) coated or otherwise deposited atop a hydrophilic substrate.
In preparing a conventional lithographic plate for use on a printing press, the plate is first exposed to actinic radiation. Specific chemical reactions are caused to occur in the plate's photoresist by exposure to actinic radiation. Such photoinduced chemical reactions may either reduce or enhance the solubility of the photoresist, depending on whether the resist is negative-working or positive-working. In negative-working plates, exposure to actinic radiation will generally cause a "hardening" of the photoresist. In positive-working plates, exposure to actinic radiation will generally cause a "softening" or "solubilization" of the photoresist.
After photoexposure, a wet development step is normally conducted. The objective of such wet development is to provide preferential solvation of the areas of the photoresist which have undergone photoinduced chemical change. Solvation under conventional development techniques will typically involve treating the exposed plate with organic solvents in a developing bath. For negative-working resists, the solvent will swell and dissolve the unexposed portions of the resist. The solvent should not swell the exposed portions or distortion of the developed image may result. For positive-working resists, the response of the unexposed and exposed coatings are reversed, but the same general principles apply.
As a result of the preferential solvation and washing away of portions of the photoresist, corresponding portions of the underlying hydrophilic substrate are uncovered. For negative-working plates, the aforementioned hydrophobic image areas correspond to the portions of the photoresist remaining after solvation and washing. The aforementioned hydrophilic non-image areas correspond to uncovered portions of the substrate. The image and non-image areas thus differentiated, the processed plate may then be mounted onto a printing press and run.
Encumbered by required wet development, the processing of conventional lithographic plates prior to their use on a printing press is time and labor consuming and involves the use of substantial quantities of organic chemicals. It will be appreciated that there is considerable attractiveness for innovations that would satisfactorily eliminate or reduce conventional lithography's long-felt dependency upon the conduct of wet development and thereby permit the use of lithographic plates on a printing press immediately after exposure without required intermediary processing.
In the past, dry-developable lithographic printing plates have been suggested which enable the wet processing steps of lithographic printing plates after exposure to be omitted and printing to be conducted by directly mounting the exposed plates on a printing press. Among printing plates that may be characterized as "on-press" developable (or related thereto) are: e.g., U.S. Pat. No. 4,273,851, issued to Muzyczko et al. on Jun. 16, 1981; U.S. Pat. No. 4,879,201, issued to Hasegawa on Nov. 7, 1989; U.S. Pat. No. 4,916,041, issued to Hasegawa et al. on Apr. 10, 1990; U.S. Pat. No. 4,999,273, issued to Hasegawa on Mar. 12, 1991; and U.S. Pat. No. 5,258,263, issued to Z. K. Cheema, A. C. Giudice, E. L Langlais, and C. F. St. Jacques on Nov. 2, 1993.
Despite the methodologies and approaches embodied in the aforementioned patents, there is a continuing need for a lithographic printing plate that can be readily developed on a printing press and that produces a plate having durable image areas needed for good run length. Difficulty in the realization simultaneously of both "on press developability" and "durability" is believed to originate from an apparent contradiction between photoresist removability ("developability") on the one hand and "durability" on the other: To make a photoresist more durable was to make the photoresist less developable.
The present invention seeks to more closely align the competing goals of "durability" and "developability" by utilizing photoreactive, polymeric binders capable of effectively functioning as both a matrix and as a photoreactive component.
In conventional lithographic printing plates, polymeric binders are incorporated into the photoresist to provide film forming properties during coating and to act as the physical backbone or matrix for the resulting photopolymerized structure resulting from exposure. Since conventional wet development techniques are oftentimes based upon the use of strong solvents, tough and comparatively durable polymeric binders may be utilized. The dictates of more narrowly defined parameters for "on press developability", govern latitude in choice of photoresist components, in general, and effectively preclude the use of tough and durable binders or other components that may not be removable under the conditions of a lithographic printing operation.
By way of illustration, in the U.S. patent application Ser. No. 08/146,710 cross-referenced above and entitled "On-Press Developable Lithographic Printing Plates", on-press development is effectuated by the use of high-boiling point, low-vapor pressure developers. These comparatively weak developers solubilize portions of the photoresist such that they may be easily washed away "on-press" by fountain and ink solutions. In this, and like systems, binders that are less durable but more easily dispersed in fountain and ink solutions are favored. However, with such binders, durability is compromised.
In order to compensate for the losses in durability, a photoresist may be designed thicker and with a larger proportion of binder to prevent tackiness. An increase in binder, however, would normally be balanced by a proportionate decrease in photopolymerizable monomers; a concomitant loss of photosensitivity would be expected. It has been found that a printing plate that can be developed effectively "on-press" and which provides image areas of good durability can be realized by utilizing in a photoresist layer thereof a polymeric binder having both structural and photoreactive capabilities. While developed in the context of "on-press" developable printing plates, several diverse applications of the inventive photoreactive polymeric binder are envisioned and enabled by the present disclosure.